The present invention relates to a holographic direct-view display which comprises at least one controllable spatial light modulator with a matrix of modulator cells for diffracting light and an array of apodisation masks. Further, this invention relates to an iterative process for finding an apodisation function for apodisation masks.
The field of application of this invention includes opto-electronic display devices which shall have a large display area and/or little structural depth, such as direct-view displays for PC, TV, mobile telephones or other appliances with the function of displaying information.
The matrix of modulator cells of a controllable spatial light modulator (SLM) comprises actively switchable modulator regions and inactive regions in between (division bars, cell boundaries). The area ratio of these two regions is known as fill factor. The inactive regions form a fixed grating structure at which light is diffracted. The diffracted light will show characteristic multi-beam interference effects if the spatial light modulator is illuminated with coherent or partly coherent light. The diffraction far field of a spatial light modulator corresponds with the Fourier transform of the complex amplitude transmission or complex amplitude reflection of the spatial light modulator. The inactive regions are predefined and characteristic for each type of SLM. At these regions the diffraction causes higher diffraction orders to occur, which can be to the detriment of the functionality and quality of an optical system.
If they are superposed on the actual image of the holographic reconstruction, higher diffraction orders can for example adversely affect the functionality of optical systems in the form of increased noise, twin images or e.g. by bright spots around the image points of the SLM which are generated in the far field. An efficient means for blanking out higher diffraction orders, which are caused by the modulator cells of the SLM matrix, which is known in the prior art are spatial filters which are disposed in an intermediate focal point of the optical system which follows the SLM. The spatial filter only transmits the desired diffraction order while all other diffraction orders are blocked by the spatial filter, which is designed as an aperture mask. A disadvantage of such a filter arrangement is that an intermediate image or an intermediate focus must be created following the SLM in the optical path. First, this considerably increases the structural depth of the optical system. Secondly, the aperture (effective opening) of the subsequent optical system (a lens or mirror) must be about as large as that of the SLM. This limits the applicability of such spatial filters to relatively small SLM and thus to projection-type displays.
Apodisation is a method for optical filtering where the outer rings of an Airy disc, which represent the higher orders, are suppressed. Advantage is taken of this method for example in imaging systems for improving the image contrast at the expense of the resolution of imaging systems, in that for example a special gradient filter is disposed in the exit aperture of the optical path.
The apodisation of modulator cells can be achieved with the help of an apodisation function tSLM pixel (x,y). Generally, apodisation functions are computed in accordance with their actual usage, and realised e.g. in a mask or filter. Further, a number of known apodisation functions which can be described analytically are discussed in the literature. In addition to a cosine or triangular function, there are for example apodisation functions which are known under the names of Blackman, Hamming, or Welch functions. These apodisation functions offer solutions for general apodisation tasks.
Document DE 10 2006 030 535 A1 filed by the applicant describes the use of apodisation functions in spatial light modulators with a pixel matrix in a projection display. The apodisation of the pixel matrix is here achieved exclusively by a respective modulation of the light which illuminates the spatial light modulators. For this, a plane coherent illumination wave is modulated with a suitable function whose periodicity is matched to the pixel structure of the modulators.
In a holographic direct-view display for generating a holographic reconstruction, the controllable light modulator is illuminated with sufficiently coherent light and generates a separate visibility region (also known as observer window) for each eye in the far field. The intensities of the higher diffraction orders can be emitted into the neighbouring visibility region and thus disturb the observer when watching the reconstruction. There are hitherto no known solutions based on apodisation which serve to reduce cross-talking of higher diffraction orders among these visibility regions.
As is generally known, to become effective, apodisation must satisfy the boundary conditions which are given by the actually used light modulator means.
Summarising, the prior art exhibits the following deficiencies. For the generation of a true holographic reconstruction, where the brightness values are represented as realistic as possible, it is required that higher diffraction orders are specifically reduced in at least one certain region. These regions lie at a defined position in the observer plane. In particular, it shall be possible to reduce particularly greatly those diffraction orders which fall into the other eye.
Another application which is not realised with conventional apodisation functions is the increase of the relative luminous intensity in at least one diffraction order other than the zeroth order relative to all other diffraction orders.